Digital lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g., red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects.
In many conventional lighting arrangements, a mechanical wall switch is used to turn ON or OFF a lighting unit by means of making or breaking an electrical connection between a load that includes the lighting unit, and a “hot” wire carrying power from the AC mains power source. Accordingly, the mechanical wall switch does not need a connection to the neutral wire from AC mains in order to turn ON and OFF the lighting unit, but instead only has an input terminal for being connected to the “hot” wire carrying power from the AC mains power source, and output terminal for supplying this power to the load when the switch turns ON the lighting unit (for safety reasons, the mechanical wall switch may also have a ground wire which does not supply any power to the wall switch or the load and which is connected to earth ground). As a result, in many existing buildings, the neutral wire from the AC mains power source is not provided to the junction box or other location where the mechanical wall switch is provided, but instead only the “hot” wire, and a wire to the load, are provided to this location (again, for safety reasons, a ground wire which does not supply any power to the wall switch or the load may also be provided and connected to earth ground). The load may include one or more lighting units, each of which may include a lighting driver and one or more light sources, such as an incandescent lamp, a fluorescent lamp (such as a compact fluorescent bulb), one or more LEDs. The load also may or may not include a ballast.
As energy saving requirements become more stringent, along with the need for intelligent lighting systems, more and more electronic controllers which employ electronic switching and dimming capabilities are deployed in place of simple mechanical wall switches in residential and commercial installations. The operation of such an electronic controller is similar to that of a mechanical wall switch, but due to the electronic circuit inside the lighting controller the electronic controller may execute additional functions, such as switching on or off a relay, dimming, switching on or off and/or dimming according to programmed timing, switching on or off and/or dimming according to various sensor inputs, wireless communications, etc. So, unlike a simple mechanical wall switch, the electronic lighting controller requires some energy for proper operation, e.g., even when the load is off.
For example, an Occuswitch Wireless Control System, available from Koninklijke Philips Electronics N.V, is an energy-saving occupancy sensor system that automatically turns lights off in an unoccupied room. As for the electronic controllers mentioned above, the Occuswitch Wireless Control System is a neutral-less electronic lighting controller, and behaves like a voltage feed when in a switch OFF state and like a current feed supply when in a switch ON state.
A neutral-less electronic lighting controller generally needs a small leakage current during the OFF state (removing power from the load) and a minimum current during the ON state (providing power to the load). However different loads have different characteristics, making it difficult to maintain a steady power supply. For example, when load impedance of a ballast is relatively large during the OFF state, the leakage current of the electronic lighting controller can develop sufficient voltage to cause the ballast to start up, which may cause lighting units to flash. During the ON state, the load needs to draw sufficient current to supply the neutral-less electronic lighting controller. Generally most ballasts have a start time during which the supply capacitors charge up and the ballasts draw very little current during this time. Also, during a preheat phase of programmed start ballasts, for example, the ballasts draw very little current. This will cause the neutral-less controller to dip during this time.
However, if the electronic controller is connected in place of a mechanical wall switch in front of the load, the maximum available power for the electronic controller is determined by the leakage current and the characteristics of the load, which is in series with the electronic controller. In some cases, for example those involving a dimming ballast having very limited leakage current, there is not a sufficient leakage current passing through the electronic controller when the load is turned OFF to keep the electronic switch operating properly. As a result, the lighting system may not operate properly.
FIG. 1 is a block diagram for a conventional lighting control system 100 which illustrates the issue. Lighting control system 100 includes a load 120 and an electronic controller 130.
The load 120 may include one or more lighting units and/or a motor (e.g., for a room fan). The lighting unit(s) may include lighting units each may include a lighting driver and one or more light sources, such as an incandescent lamp, a fluorescent lamp (such as a compact fluorescent bulb), one or more light emitting diodes (LEDs), etc. The load 120 also may or may not include a ballast. The load 120 has the first load terminal and a second load terminal, and is configured to receive a load voltage between the first and second load terminals and is further configured to allow a load current to flow between the first and second load terminals.
Electronic controller 130 has a single input terminal connected via a wire (e.g., a black wire) to a first power terminal 110 of an external power source 105 (e.g., AC mains), which outputs an AC voltage between first power terminal 110 and a second power terminal (e.g., a neutral terminal) 112 thereof. Also shown is a ground wire (e.g., a green wire) 114 which is connected to earth ground and which does not supply any power to the electronic controller 130 or the load 120. The electronic controller 130 also has a single output terminal which is connected by a wire (e.g., a red wire) to the first load terminal of the load 120. The second load terminal of the load 120 is connected by a wire (e.g., a neutral wire, which may be a white wire) to the second power terminal 112 of the external power source 105.
When the electronic controller 130 is in the ON state so as to power the load 120, then the load 120 can receive as its load voltage 100% of the input voltage supplied from the external power source 105. When the electronic controller 130 is in the OFF state so as to disable the load 120, then the load voltage across the load 120 will be zero.
However, since the electronic controller 130 is an electrical device which requires power to operate, the situation can become complicated. When electronic the controller 130 is in the ON state, if the load voltage across the load 120 is 100% of the input voltage supplied from the external power source 105, then the voltage across the electronic controller 130 will be zero, and it can not remain in the ON state for long. Meanwhile, when the electronic controller 130 is in the OFF state, there will be no load voltage across the load 120 and no load current flowing through the load 120. However this means that there will also be no current, or very little current, passing through the electronic controller 130, so it cannot maintain the OFF state either, if it requires more energy.
To address these issues, some electronic controllers are designed to modulate the time intervals when they are in the ON and OFF states. When the electronic controller is in the ON state, it will switch to the OFF state for a little while, (e.g., OFF for 2 ms during every 10 ms ON period), so that during this interval the electronic controller can receive 100% of the input voltage supplied from the external power source 105 and thereby power itself. Meanwhile, when the electronic controller is in OFF state, it maintains a small leakage current flowing through the load, and with such leakage current, the electronic controller can power itself as well.
However, along with the technology development and more features like wireless communication required for lighting control, power consumption of an electronic controller increases significantly, and the intrinsic leakage current of the load itself is not sufficient to power the electronic controller when it is in the OFF state.
FIG. 2 is a block diagram for another conventional lighting control system 200 which has been provided to try to address this issue. The lighting control system 200 is identical to the lighting control system 100, except that the lighting control system 200 includes an external capacitor 210 connected across the load terminals of the load 120. Whether the electronic controller 130 is in an ON state or an OFF state, the external capacitor 210 can provide a leakage current path for the electronic controller 130. The bigger the capacitor, the more leakage current can be delivered to the electronic controller 130 to support activities consuming much current and power (e.g., receiving a wireless control signal).
However, if the electronic controller 120 includes a TRIAC based device, also known as leading edge dimmer, then the external capacitor 210 will cause catastrophic damage to TRIAC in terms of huge inrush current every cycle. Additionally, the external capacitor 210 will shift the phase of voltage and current at the load side, making the phase cutting of the dimming operation out of control.
Thus, it would be desirable to provide a lighting control system which can supply a necessary leakage current to a controller when the controller is in an OFF state and disables a load having power supplied by the controller. It would be further desirable to provide a lighting control system which can supply a necessary leakage current to a controller when the controller initially transitions to the ON state, while the load having power supplied by the controller begins to draw sufficient current for operation of the controller.